Materials

Question 1

What is the density of a material?

Answer 1

Its mass per unit volume. It is a measure of how compact a substance is.

Question 2

What does Hooke’s law state?

Answer 2

The extension is directly proportional to the force applied, given that the environmental conditions (i.e. temperature) are kept constant.

Question 3

What is the limit proportionality?

Answer 3

The point after which Hooke’s law is no longer obeyed.

Question 4

What is the elastic limit?

Answer 4

The point (just after the limit of proportionality) where if you increase the force applied, the material will deform plastically (permanently stretched).

Question 5

What is P and E?

Answer 5

P = limit of proportionality

E = elastic limit

Question 6

What is the equation for Hooke’s law?

Answer 6

F = Kx

F = force

K = spring constant

X = extension

Question 7

What is tensile stress?

Answer 7

Force applied per unit cross sectional area.

Stress = F / A

Question 8

What is tensile strain?

Answer 8

The extension caused by the tensile stress over the original length.

Strain = x / L

Question 9

How can you calculate the elastic strain energy in a force-extension graph?

Answer 9

Calculate the area under the graph:

1/2.F.X

Question 10

What is the breaking stress?

Answer 10

The value of stress at which the material will break.

This depends on conditions (temperature, humidity…)

Question 11

What are the properties of a plastic material?

Answer 11

Large amount of extension as load is increased, especially beyond the elastic limit.

Question 12

What are the properties of a brittle material?

Answer 12

The material will extend very little, and therefore will break at a low extension.

Question 13

What would we see in a force-extension graph of a material beyond its elastic limit, being loaded and unloaded?

Answer 13

The unloading line will not return to the origin.

However, the lines would be parallel since K, the material stiffness is constant.

The area between the two lines is the work done to permanently deform the material.

Question 14

How is energy stored in elastic and plastic stretches?

Answer 14

Elastic: all the work is stored as elastic strain energy,

Plastic: work done is used to move atoms, so energy is dissipated as heat.

Question 15

How do the energy stores vary when a load is hung and removed from an elastic spring?

Answer 15

Kinetic (stretching) - elastic strain (stretched) - kinetic (unstretching) - gpe

Question 16

What is the difference between stress-strain graphs and force-extension graphs?

Answer 16

Stress-strain graphs describe the behaviour of a material rather than a specific object.

Question 17

What is UTS?

Answer 17

Ultimate tensile stress - highest point in a stress-strain graph as it shows the maximum stress the material can withstand.

Question 18

What type of material is A?

Answer 18

Brittle

Question 19

What type of material is B?

Answer 19

Ductile

Question 20

What type of material is C?

Answer 20

Plastic

Question 21

What is Young modulus?

Answer 21

A value which describes the stiffness of a material.

Young modulus (E) = tensile stress / tensile strain

= F/A / X/L = F.L / X.A